JWITC 2013Jan. 19, On the Capacity of Distributed Antenna Systems Lin Dai City University of Hong Kong

JWITC 2013Jan. 19, Cellular Networks (1) Multiple antennas at the BS side Base Station (BS) Growing demand for high data rate

JWITC 2013Jan. 19, Cellular Networks (2) Co-located BS antennasDistributed BS antennas Implementation cost Sum rate Lower Higher?

JWITC 2013Jan. 19, 2013 A little bit of History of Distributed Antenna Systems (DAS) Originally proposed to cover the dead spots for indoor wireless communication systems [Saleh&etc’1987]. Implemented in cellular systems to improve cell coverage. Recently included into the 4G LTE standard. Multiple-input-multiple-output (MIMO) theory has motivated a series of information-theoretic studies on DAS. 4

JWITC 2013Jan. 19, 2013 Single-user SIMO Channel Single user with a single antenna. L>1 BS antennas. Uplink (user->BS). (a) Co-located BS Antennas (b) Distributed BS Antennas 5 Received signal: Channel gain :: Large-scale fading : Small-scale fading : Transmitted signal : Gaussian noise

JWITC 2013Jan. 19, 2013 Single-user SIMO Channel Ergodic capacity without channel state information at the transmitter side (CSIT): Single user with a single antenna. L>1 BS antennas. Uplink (user->BS). Ergodic capacity with CSIT: Function of large-scale fading vector  (a) Co-located BS Antennas (b) Distributed BS Antennas 6

JWITC 2013Jan. 19, 2013 Co-located Antennas versus Distributed Antennas With co-located BS antennas: Ergodic Capacity without CSIT:  is the average received SNR: With distributed BS antennas: Distinct large-scale fading gains to different BS antennas. Ergodic Capacity without CSIT: Normalized channel gain: 7

JWITC 2013Jan. 19, 2013 Capacity of DAS 8 For given large-scale fading vector  : [Heliot&etc’11]: Ergodic capacity without CSIT o A single user equipped with N co-located antennas. o BS antennas are grouped into L clusters. Each cluster has M co-located antennas. o Asymptotic result as M and N go to infinity and M/N is fixed. [Aktas&etc’06]: Uplink ergodic sum capacity without CSIT o K users, each equipped with N  k co-located antennas. o BS antennas are grouped into L clusters. Each cluster has N l co-located antennas. o Asymptotic result as N goes to infinity and  k and l are fixed. Implicit function of  (need to solve fixed-point equations) Computational complexity increases with L and K.

JWITC 2013Jan. 19, 2013 Capacity of DAS [Choi&Andrews’07], [Wang&etc’08], [Feng&etc’09], [Lee&ect’12]: BS antennas are regularly placed in a circular cell and the user has a random location. 9 Average ergodic capacity (i.e., averaged over  ) [Zhuang&Dai’03]: BS antennas are uniformly distributed over a circular area and the user is located at the center. [Roh&Paulraj’02], [Zhang&Dai’04]: The user has identical access distances to all the BS antennas. High computational complexity! With random large-scale fading vector  :  Single user  Without CSIT

JWITC 2013Jan. 19, 2013 Questions to be Answered How to characterize the sum capacity of DAS when there are a large number of BS antennas and users? How to conduct a fair comparison with the co-located case? Large-system analysis using random matrix theory. What is the effect of CSIT on the comparison result? Decouple the comparison into two parts: 1) capacity comparison and 2) transmission power comparison for given average received SNR. K randomly distributed users with a fixed total transmission power. 10 Bounds are desirable.

JWITC 2013Jan. 19, Part I. System Model and Preliminary Analysis [1] L. Dai, “A Comparative Study on Uplink Sum Capacity with Co-located and Distributed Antennas,” IEEE J. Sel. Areas Commun., 2011.

JWITC 2013Jan. 19, 2013 Assumptions K users uniformly distributed within a circular cell. Each has a single antenna. L BS antennas. Uplink (user->BS). (a) Co-located Antennas (CA) (b) Distributed Antennas (DA) * : user o: BS antenna 12 Random BS antenna layout!

JWITC 2013Jan. 19, 2013 Uplink Ergodic Sum Capacity Received signal: : Transmitted signal : Gaussian noise : Large-scale fading : Small-scale fading : Channel gain Uplink power control: Ergodic capacity without CSITErgodic capacity with CSIT 13

JWITC 2013Jan. 19, 2013 More about Normalized Channel Gain Normalized channel gain vector: : Normalized Large-scale fading : Small-scale fading With CA: With DA: o With a large L, it is very likely that user k is close to some BS antenna : The channel becomes deterministic with a large number of BS antennas L! Channel fluctuations are preserved even with a large L! 14

JWITC 2013Jan. 19, 2013 More about Normalized Channel Gain Theorem 1. For n=1,2,…, which is achieved when Channel fluctuations are minimized when maximized when Channel fluctuations are undesirable when CSIT is absent, desirable when CSIT is available. 15

JWITC 2013Jan. 19, 2013 Single-user Capacity (1) (average received SNR) Without CSIT -- Fading always hurts if CSIT is absent! quickly approaches 1 as L grows. With CSIT ( when  0 is small ) at low  0 --“Exploit” fading 16

JWITC 2013Jan. 19, 2013 Single-user Capacity (2) The average received SNR. Without CSIT With CSIT A higher capacity is achieved in the CA case thanks to better diversity gains. A higher capacity is achieved in the DA case thanks to better waterfilling gains. 17 DA with

JWITC 2013Jan. 19, Part II. Uplink Ergodic Sum Capacity

JWITC 2013Jan. 19, 2013 Uplink Ergodic Sum Capacity without CSIT Sum capacity without CSIT: Sum capacity per antenna (with K>L): where denotes the eigenvalues of. 19

JWITC 2013Jan. 19, 2013 More about Normalized Channel Gain Theorem 2. As and when andwith when With DA and as and With CA: as and [Marcenko&Pastur’1967] 20

JWITC 2013Jan. 19, 2013 Sum Capacity without CSIT (1) Gap diminishes when  is large -- the capacity becomes insensitive to the antenna topology when the number of users is much larger than the number of BS antennas. (average received SNR) 21

JWITC 2013Jan. 19, 2013 Sum Capacity without CSIT (2) The average received SNR A higher capacity is achieved in the CA case thanks to better diversity gains. The number of users K=100. serves as an asymptotic lower-bound to. 22 DA with CA

JWITC 2013Jan. 19, 2013 Uplink Ergodic Sum Capacity with CSIT Sum capacity with CSIT: With CA: The optimal power allocation policy: where  is a constant chosen to meet the power constraint, k=1,…, K. [Yu&etc’2004] With DA and The optimal power allocation policy: i=1,…,L, where  is a constant chosen to meet the sum power constraint 23

JWITC 2013Jan. 19, 2013 Signal-to-Interference Ratio (SIR) (a) CA(b) DA with The received SNRThe number of users K=100. The number of BS antennas L=10. 24

JWITC 2013Jan. 19, 2013 Sum Capacity (1) (average received SNR) Without CSIT With CSIT Gap between and even at high SNR (i.e., thanks to better multiuser diversity gains) is enlarged as L grows (i.e., due to a decreasing K/L). 25 The number of users K=100.

JWITC 2013Jan. 19, 2013 Sum Capacity (2) The average received SNRThe number of users K=100. Without CSIT With CSIT A higher capacity is achieved in the CA case thanks to better diversity gains. A higher capacity is achieved in the DA case thanks to better waterfilling gains and multiuser diversity gains. 26 DA with CA

JWITC 2013Jan. 19, Part III. Average Transmission Power per User

JWITC 2013Jan. 19, 2013 Average Transmission Power per User Transmission power of user k: With DA: What is the distribution of Average transmission power per user: o Both users and BS antennas are uniformly distributed in the circular cell. With CA: o Users are uniformly distributed in the circular cell. BS antennas are co-located at cell center. 28

JWITC 2013Jan. 19, 2013 Minimum Access Distance With DA, each user has different access distances to different BS antennas. Let denote the order statistics obtained by arranging the access distances d 1,k,…, d L,k. for L>1. An upper-bound for average transmission power per user with DA: 29

JWITC 2013Jan. 19, 2013 Average Transmission Power per User CA: Path-loss factor  =4. DA: (path-loss factor  >2) With  =4, if For given received SNR, a lower total transmission power is required in the DA case thanks to the reduction of minimum access distance. 30

JWITC 2013Jan. 19, 2013 Sum Capacity without CSIT The number of users K=100. For fixed K and ( such that 31 ) Given the total transmission power, a higher capacity is achieved in the DA case. Gains increase as the number of BS antennas grows.

JWITC 2013Jan. 19, 2013 Conclusions A comparative study on the uplink ergodic sum capacity with co- located and distributed BS antennas is presented by using large- system analysis. –A higher sum capacity is achieved in the DA case. Gains increase with the number of BS antennas L. –Gains come from 1) reduced minimum access distance of each user; and 2) enhanced channel fluctuations which enable better multiuser diversity gains and waterfilling gains when CSIT is available. Implications to cellular systems: –With cell cooperation: capacity gains achieved by a DAS over a cellular system increase with the number of BS antennas per cell thanks to better power efficiency. –Without cell cooperation: lower inter-cell interference with DA? 32

JWITC 2013Jan. 19, 2013 Thank you! Any Questions? 33